Wireless terminal devices, such as mobile phones, have gradually become an indispensable part in modern life, and currently, demands for wireless terminal devices that support multimode and multiband are becoming more obvious. In the wireless terminal devices, radio frequency paths of different frequency bands share a same antenna, and therefore, a multimode radio frequency antenna switch becomes an important part in a radio frequency front module of the wireless terminal devices.
A single-pole double-throw (SPDT) switch is used as an example to describe a structure of a commonly used radio frequency antenna switch. As shown in FIG. 1, FIG. 1 is a schematic diagram of a basic structure of an SPDT radio frequency antenna switch. The SPDT radio frequency antenna switch includes two paths, and each path includes a series branch (M1, M2) and a parallel branch (M3, M4). M1 to M4 are all transistor devices. A branch connecting a signal end and an antenna is a series branch, for example, M1 in the series branch is used to send a signal from a power amplifier (PA) to the antenna. A branch connecting the signal end and the ground is a parallel branch, for example, when M1 in the series branch is conducted, in order to send, to the antenna, a signal at a transmit end from a PA output end, M3 in the parallel branch at the transmit end and M2 in the series branch at a receive end should be in an off state, while M4 in the parallel branch at the receive end should be in an on state, so as to short-circuit a signal transmitted to a low noise amplifier (LNA) input end to ground. A resistor Rs or Rp connected to a gate of each transistor is a gate isolation resistor, which is configured to reduce a leakage current of the gate.
In a Global System for Mobile Communication (GSM) application, for a low frequency signal in the case of match, a maximum amplitude of an input signal can reach 20 volts (V); however, in a case of mismatch, the voltage amplitude can reach 40 V. As a result, reliability of a transistor device in a branch in an off state is seriously affected due to a limited breakdown voltage Vbreakdown of a transistor.
To solve a reliability problem of the transistor device, a stack manner is generally used in the other approaches, that is, multiple transistors are stacked or cascaded. As shown in FIG. 2A and FIG. 2B, FIG. 2A is a schematic principle diagram showing that reliability of a transistor device in a turned-off branch is improved in a stack manner; and FIG. 2B is a schematic diagram showing unbalanced voltage distribution caused by a current leakage from a gate to a body of a transistor in a stack manner. As shown in FIG. 2A, when a transistor at an antenna transmit end, for example, M1 in FIG. 1, is in an on state ON, a sine voltage vTX is input, and in this case, n stacked transistors in a branch at a receive end are in an off state, for example, for M2 in FIG. 1 implemented by stacking n transistors, in an ideal case, a voltage of vTX/n is evenly borne between a source and a drain of each of the n stacked transistors, thereby reducing a voltage distributed to each transistor, and further facilitating improvement of transistor reliability. Each transistor in an off state may be equivalent to two parasitic capacitors Cgs and Cgd being in series connection, and therefore, it is equivalent that Cgs or Cgd of each transistor bears a distributed voltage of vTX/2n. However, due to an actual problem of a manufacturing process of a transistor, a current leakage may exist at a gate and a body thereof, and current leakages at gates and bodies of all transistors are uneven, which causes that voltage distribution among the n stacked transistors is uneven. In this case, even if a value of n is properly selected according to vTX/2n<Vbreakdown, the reliability is still affected. In FIG. 2B, Ig is a gate current, and Isub is a body current. Due to existence of Ig and Isub, from a transistor close to the antenna, source-drain currents Id1 to Idn of the transistors are different. To solve a problem that uneven voltage distribution among the stacked transistors causes poor transistor reliability, a solution is provided in the other approaches. As shown in FIG. 3, FIG. 3 is a schematic diagram of a circuit in which voltage distribution is balanced in another approach. In FIG. 3, a gate of each of the stacked transistors (M1 to MN) is connected to a high-resistance resistor Rg, and the other end of the resistor is short-circuited to a common point G. In addition, a substrate, referred to as a body, of each transistor, is connected to a high-resistance resistor Rb, and the other end of the resistor is short-circuited to a common point B. To overcome the problem that uneven voltage distribution among the transistors cause poor transistor reliability, as shown in FIG. 3, a coupling capacitor (Cgg1 to Cgg(N−1)) or a coupling resistor (Rgg1 to Rgg(N−1)) or a network (Cgg1 Rgg1 to Cgg(N−1) Rgg(N−1)) in which a capacitor is connected in series with a resistor is added between gates of each of the stacked transistors, or a coupled circuit is added between a main signal path (a path formed by cascading M1 to MN) and a bias circuit (a path formed by cascading Rgs or a path formed by cascading Rbs), where the coupled circuit may be implemented using a single coupling capacitor, for example, Cfwd, or may be implemented using a circuit in which a capacitor is connected in series with a resistor, for example, Cfwd and Rfwd.
To solve the reliability problem of the transistor device, another solution is also provided in the other approaches. As shown in FIG. 4, FIG. 4 is a schematic diagram of another circuit in which voltage distribution is balanced in another approach. Because in the a case of a large signal voltage swing, some transistors in a branch in an off state are probably conducted, for example, transistors M1 to M3 shown in (a) and transistors M1 to M2 shown in (b) in FIG. 4. In this way, a large signal swing is distributed to another transistor that is turned off, for example, a transistor M4 shown in (a) and transistors M3 to M4 shown in (b) in FIG. 4. Because the total number of transistors to which the voltages are evenly distributed is reduced, a voltage borne by the turned-off transistor M4 or the turned-off transistors M3 to M4 increases, which causes the reliability problem. Therefore, in solution (a) shown in FIG. 4, a body B1 (B2, B3) and a source S1 (S2, S3) of the transistors M1 to M3 which are close to an antenna end in a branch in an off state are short-circuited, and in solution (b) shown in FIG. 4, a body B1 (B2) and a source S1 (S2) of the transistors M1 to M2 close to an antenna end in a branch in an off state are short-circuited, so that in a negative half cycle of the large signal voltage swing, a junction diode between a body and a source of each of the stacked transistors is not conducted, and more transistors are made to share the voltage; however, a connection between a source and a body introduces an extra signal path to a main signal path.
However, in the foregoing solutions in the other approaches, an extra passive device, such as a coupling capacitor or a resistor, needs to be added, or an extra signal path is introduced to a main signal path, which not only causes an increase of costs, but more importantly, causes the foregoing coupling device and signal path tend to provide an extra transmission path for a signal to be transmitted, thereby causing a signal leakage, which results in performance deterioration such as an insertion loss of a switch.